1. Technical Field
This invention relates to a precipitation process for sodium aluminate solutions. In one aspect, this invention relates to a precipitation process for precipitating alumina hydrate from supersaturated sodium aluminate solutions.
2. Background
The Bayer process is a chemical process for refining alumina. The Bayer process takes advantage of several chemical and physical phenomena. Aluminum hydroxide contained in bauxite ore reacts readily with sodium hydroxide under moderate digestion conditions to form sodium aluminate. This reaction reverses at lower temperatures to hydrolyze the sodium aluminate to form Al(OH).sub.3 and regenerate the NaOH, in the presence of aluminum hydroxide seed.
In the Bayer process, the bauxite ore first is prepared by grinding, blending, and slurrying. The prepared bauxite then is brought into contact in a chemical digester vessel with a hot solution of NaOH, which reacts with the Al(OH).sub.3 or AlOOH in the bauxite to form a solution of NaAlO.sub.2. Impurities such as oxides, phosphates, and carbonates are precipitated as relatively insoluble compounds. The slurry leaving the digester at above its atmospheric boiling point is flashed to lower pressures. Steam evolved in flashing heats the process liquor as it is pumped to the digester. A solid bauxite residue is separated from the sodium aluminate solution.
The sodium aluminate solution then is cooled further so that the Al(OH).sub.4.sup.- ions go through dehydroxylation to form Al (OH).sub.3 in precipitation, reversing the reaction that took place in digestion. The Al(OH).sub.3 solids produced in precipitation are classified into a coarse product fraction and finer material. The fines are returned to precipitation for agglomeration and growth. The coarse Al(OH).sub.3 fraction is washed and calcined to produce the Al.sub.2 O.sub.3 product.
Prior to the precipitation unit operation of the Bayer process, alumina has been dissolved by the digestion of bauxite, and most of the impurities from the bauxite have been removed in clarification. In precipitation, the alumina is recovered as Al(OH).sub.3 crystals grown to particles having the proper size and characteristics for calcination to smelting grade alumina.
In alumina production, the precipitation operation is somewhat unusual technically in that no chemical reagent is added to the solution to form a precipitate, and the operation is not true crystallization because the soluble compound, NaAlO.sub.2, does not come out of solution. Instead, the reaction goes as follows: EQU Na.sup.+ +Al(OH).sub.4.sup.- .fwdarw.Al(OH).sub.3 +Na.sup.+ +OH.sup.- Equation (1)
The structural formula for the aluminate ion is either Al(OH).sub.4.sup.- or Al(OH).sub.4.sup.-.2 H.sub.2 O in solutions having concentrations in the range normal to precipitation. The sodium ion appears unchanged on both sides of Equation (1) but is included in Equation (1) to depict that a moderately concentrated solution of caustic is necessary to the process.